Double row tapered roller bearing, bearing ring, and method for producing double row tapered roller bearing

ABSTRACT

A double row tapered roller bearing includes: an outer ring having an annular shape; an inner ring disposed on an inner circumferential side of the outer ring and having an annular shape; and rollers. The inner ring has an outer circumferential surface facing the outer ring and having two rows of grooves having a bottom surface serving as a raceway surface. The rollers are tapered rollers disposed in the grooves in contact with the raceway surface of the inner ring and are also in contact with the outer ring. At outer circumferential surface of the inner ring, a region adjacent to the groove includes a hardened region extending from the inner peripheral surface of the groove to the region adjacent to the groove, and an unhardened region located at a position farther from the groove than the hardened region and being smaller in hardness than the hardened region.

CROSS REFERENCE

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/JP2016/080240, filed on Oct. 12, 2016,which claims the benefit of Japanese Application No. 2015-213092, filedon Oct. 29, 2015, the entire contents of each are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to a double row tapered roller bearing, abearing ring, and method for producing the double row tapered rollerbearing.

BACKGROUND ART

A bearing for a wind turbine generator, such as a main shaft bearingthat supports a shaft transmitting a blade's rotational power, is actedon not only by a load component attributed to the weight of the bladeand that of a rotor but also by a load component attributed to a windload. That is, in addition to a radial load, an axial load also acts onthe bearing. For this reason, it has been conventionally proposed to usea double row tapered roller bearing as a bearing for a wind powergenerator (see, for example, Japanese Patent National Publication No.2008-546948).

CITATION LIST Patent Document

PTD 1: Japanese Patent National Publication No. 2008-546948

SUMMARY OF INVENTION Technical Problem

As disclosed in Japanese Patent National Publication No. 2008-546948, adouble row tapered roller bearing applied to a wind power generator hasan outer ring with a plurality of bolt holes, and is secured to ahousing of the wind power generator by bolts inserted through the boltholes. Bolt holes may similarly be formed for an inner ring of thedouble row tapered roller bearing.

For such a double row tapered roller bearing's outer and inner rings,there is adopted a process of carburizing and quenching usingcarburizing steel in order to obtain required hardness. This is based onthe following reason:

A plurality of bolt holes formed in an outer ring or the like asdescribed above are required to have a high positional accuracy in orderto accurately fix the double row tapered roller bearing to neighboringparts. Accordingly, forming the bolt holes after a heat treatment forthe outer ring or the like is completed can eliminate the necessity ofconsidering such deformation of the outer ring accompanying the heattreatment as in a case of forming the bolt holes before the heattreatment, and can also contribute to enhanced working efficiency. Onthe other hand, the outer ring or the like increased in hardness by theheat treatment is impaired in machinability and accordingly, difficultto machine. That is, using a bearing steel as a material for an outerring and the like and performing general, entire quenching as a heattreatment make working bolt holes difficult.

Accordingly, when carburizing steel is used as a material for an outerring and the like and carburized and quenched in a state in which ananti-carburization treatment has been applied to a region in which boltholes should be formed, a region without the anti-carburizationtreatment can be enhanced in hardness, whereas the region havingundergone the anti-carburization treatment is prevented from beingincreased in hardness, and accordingly, a process for forming bolt holesafter the carburizing and quenching can be easily performed.

However, if such a carburizing heating process as described above isperformed, the number of steps including the anti-carburizationtreatment is increased to be larger than a typical quenching process,and the heating process's own processing time is also longer thangeneral, entire quenching, resulting in an increased production cost.

The present invention has been made to address the above issue, andcontemplates a double row tapered roller bearing produced at reducedcost.

Solution to Problem

A double row tapered roller bearing according to the present disclosurecomprises: an outer ring serving as a bearing ring having an annularshape; an inner ring disposed on an inner circumferential side of theouter ring and serving as a bearing ring having an annular shape; androllers. The inner ring has an outer circumferential surface facing theouter ring and having two rows of grooves having a bottom surfaceserving as a raceway surface. The rollers are tapered rollers disposedin the grooves in contact with the raceway surface of the inner ring andare also in contact with the outer ring. At the outer circumferentialsurface of the inner ring, a region adjacent to the groove includes ahardened region extending from an inner peripheral surface of the grooveto the region adjacent to the groove, and an unhardened region locatedat a position farther from the groove than the hardened region and beingsmaller in hardness than the hardened region.

A method for producing a bearing ring according to the presentdisclosure is a method for producing a bearing ring of a double rowtapered roller bearing, comprising the steps of: preparing a formedbody; forming a heated region; cooing; and removing. In the step ofpreparing a formed body, there is prepared a formed body constituted ofsteel and having an outer circumferential surface having an annulargroove having a bottom surface to serve as a raceway surface of thebearing ring. In the step of forming a heated region, the formed body isinduction heated to form a heated region including the bottom surface ofthe groove and heated to a temperature of at least an A₁ point. In thecooling step, the whole of the heated region is simultaneously cooled toa temperature of not more than an M_(s) point. In the step of preparinga formed body, the formed body includes an excessive portion in which aregion adjacent to the groove extends outwardly of a position whichshould be an outer circumferential surface of the bearing ring. In thestep of removing, the excessive portion is removed from the formed bodyafter the step of cooling.

A method for producing a double row tapered roller bearing according tothe present disclosure includes the steps of: preparing a bearing ring;preparing tapered rollers; and assembling a double row tapered rollerbearing by combining the bearing ring and the rollers. The bearing ringis produced in the method for producing a bearing ring as describedabove.

Advantageous Effects of Invention

Thus a double row tapered roller bearing can be obtained that comprisesa bearing ring having sufficient characteristics without inviting anincreased production cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a double row tapered roller bearingaccording to an embodiment.

FIG. 2 is a partial cross-sectional schematic view taken along a lineII-II shown in FIG. 1.

FIG. 3 is a schematic view for illustrating a wind turbine generatorwith the FIG. 1 double row tapered roller bearing applied thereto.

FIG. 4 is a flow chart generally representing a method for producing abearing ring of the double row tapered roller bearing shown in FIG. 1and the double row tapered roller bearing.

FIG. 5 is a schematic cross-sectional view of a formed body.

FIG. 6 is a partial, schematic cross-sectional view of the formed body.

FIG. 7 is a schematic diagram for illustrating a quench hardening step.

FIG. 8 is a schematic sectional view taken along a line VIII-VIII shownin FIG. 7.

FIG. 9 is a schematic diagram for illustrating a finishing step.

FIG. 10 is a partial cross-sectional schematic view of a bearing ring asa comparative example.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are now described with reference tothe drawings. In the figures, identical or corresponding components areidentically denoted and will not be described redundantly.

<Configuration of Double Row Tapered Roller Bearing>

A structure of a double row tapered roller bearing according to anembodiment will be described with reference to FIGS. 1 and 2.

The double row tapered roller bearing shown in FIGS. 1 and 2 mainlycomprises: an outer ring 2 serving as a bearing ring having an annularshape; an inner ring 5 disposed on an inner circumferential side ofouter ring 2 and serving as a bearing ring having an annular shape; aplurality of rollers 6 serving as rolling elements; and a cage 7defining how the plurality of rollers 6 are disposed. A bolt hole 8 isformed in outer ring 2. Bolt hole 8 is formed to extend in a thrustdirection of the double row tapered roller bearing. Furthermore, outerring 2 has an inner circumferential surface with two raceway surfacesformed thereon. The two raceway surfaces include outer ring hardenedregions 15 a. Furthermore, a portion of outer ring 2 other than thathaving outer hardened regions 15 a is an unhardened region 18 lower inhardness than outer ring hardened regions 15 a.

Inner ring 5 includes two inner ring members 3 a and 3 b and an innerring spacer 4. Two inner ring members 3 a and 3 b each have an annularshape. Inner ring spacer 4 has an annular shape and is disposed betweeninner ring members 3 a and 3 b. Inner ring spacer 4 may be dispensedwith. Inner ring members 3 a and 3 b each have an outer circumferentialsurface 16 facing outer ring 2 and having a groove having a bottomsurface serving as a raceway surface. That is, inner ring 5 has two rowsof grooves 19. From another point of view, outer circumferential surface16 means a surface portion of inner ring member 3 a, 3 b that extendsalong the central axis of roller 6. Rollers 6 are disposed in groove 19in contact with the raceway surface of inner ring 5 and are also incontact with outer ring 2. Roller 6 is a tapered roller. At outercircumferential surface 16 of inner ring 5, a region adjacent to groove19 includes inner ring hardened region 15 b extending from the innerperipheral surface of groove 19 to the region adjacent to groove 19, andunhardened region 18 located at a position farther from groove 19 thaninner ring hardened region 15 b and being smaller in hardness than innerring hardened region 15 b. The region of outer circumferential surface16 of inner ring 5 that is adjacent to groove 19 as shown in FIG. 2 is aregion that sandwiches groove 19 in a direction along central axis 25 ofinner ring 5 and extends along the central axis of roller 6. Fromanother point of view, at outer circumferential surface 16 of inner ring5, inner ring hardened region 15 b is formed adjacent to annular groove19 along groove 19. From another point of view, a boundary portion 17between inner ring hardened region 15 b and unhardened region 18 isannularly arranged along groove 19. Inner ring hardened region 15 b isformed to extend from bottom and side surfaces of groove 19 to outercircumferential surface 16.

An angle θ that the bottom surface of groove 19 serving as the racewaysurface forms with central axis 25 of inner ring 5 is at least 40° andnot more than 50°. Further, angle θ may be 45°.

<Function and Effect of Double Row Tapered Roller Bearing>

In double row tapered roller bearing 1 shown in FIGS. 1 and 2, outercircumferential surface 16 of inner ring 5 includes unhardened region18, and machining such as drilling unhardened region 18 can be easilyperformed. Further, outer ring 2 similarly has unhardened region 18, andbolt hole 8 can be easily formed after a heat treatment for forminghardened region 15 outer ring hardened regions 15 a is performed.

In double row tapered roller bearing 1, angle θ that the bottom surfaceof groove 19 serving as the raceway surface forms with central axis 25of inner ring 5 is at least 40° and not more than 50°, and double rowtapered roller bearing 1 can alone provide a large action pointdistance. Accordingly, applying double row tapered roller bearing 1 as abearing for a main shaft of a wind turbine generator allows a bearingportion for the main shaft of the wind power generator to bedimensionally smaller than applying a plurality of cylindrical rollerbearings as a bearing for that main shaft does.

<Configuration of Wind Turbine Generator with Double Row Tapered RollerBearing Applied Thereto>

With reference to FIG. 3, a configuration of a wind turbine generator towhich the double row tapered roller bearing shown in FIG. 1 is appliedwill be described.

With reference to FIG. 3, a wind turbine generator 10 mainly includes amain shaft 22, a blade 30, a speed up gear 40, a power generator 50, anda main shaft bearing 60. Speed up gear 40, power generator 50 and mainshaft bearing 60 are housed in a nacelle 90. Nacelle 90 is supported bya tower 100. That is, nacelle 90 is provided at an upper end of tower100 of the wind turbine generator.

A plurality of blades 30 are attached to a rotor head 20 connected tothe upper end of main shaft 22. Main shaft 22 is supported insidenacelle 90. The rotation of main shaft 22 is transmitted to powergenerator 50 via speed up gear 40.

Main shaft 22 enters nacelle 90 from rotor head 20 and is connected tothe input shaft of speed up gear 40. Main shaft 22 is rotatablysupported by main shaft bearing 60. And main shaft 22 transmits rotationtorque that is generated by blade 30 receiving wind power to the inputshaft of speed up gear 40. Blade 30 converts wind power into rotationtorque, and transmits it to main shaft 22.

Main shaft bearing 60 is disposed in nacelle 90 in a fixed manner andsupports main shaft 22 rotatably. Main shaft bearing 60 is constitutedby double row tapered roller bearing 1 shown in FIGS. 1 and 2.Furthermore, double row tapered roller bearing 1 shown in FIGS. 1 and 2used as main shaft bearing 60 is fixed to nacelle 90 by bolts insertedthrough bolt holes 8 of outer ring 2 shown in FIG. 2.

Speed up gear 40 is provided between main shaft 22 and power generator50, accelerates the rotation speed of main shaft 22 and outputs it topower generator 50. As an example, speed up gear 40 is composed of agear speed-up mechanism including a planetary gear, a countershaft, ahigh speed shaft, etc. Power generator 50 is connected to an outputshaft 61 of speed up gear 40, and generates electric power by therotation torque received from speed up gear 40. Power generator 50 is aninduction generator, for example.

The wind turbine generator is configured to be capable of performing ayaw motion to rotate nacelle 90 in accordance with the wind directionwith respect to tower 100 fixed on the ground. Preferably, nacelle 90 isrotated so that the blade 30 side is positioned windward.

Further, wind turbine generator 10 may be configured to obtainappropriate rotation by changing an angle (hereinafter referred to as apitch) of blade 30 with respect to the wind direction depending on thestrength of the wind force. Furthermore, wind turbine generator 10 maybe configured to similarly control the blades' pitch when starting andstopping the wind turbine. Further, wind turbine generator 10 may alsobe configured so that each blade 30 swings by several degrees while mainshaft 22 is rotated once. By doing so, an amount of energy that can beobtained from wind can be adjusted. For example, for strong wind, theblades have a wind receiving surface (also referred to as a wingsurface) set parallel to the wind direction in order to suppress therotation of the wind turbine.

<Method for Producing Bearing Ring of Double Row Tapered Roller Bearing,and Double Row Tapered Roller Bearing>

A method for producing a bearing ring of a double row tapered rollerbearing and the double row tapered roller bearing will be described withreference to FIGS. 4 to 10. While a method for producing inner ringmember 3 a (see FIG. 2) will mainly be described as a method forproducing a bearing ring, inner ring member 3 b (see FIG. 2) and outerring 2 can also be similarly produced.

Referring to FIG. 4, a formed body preparation step is first carried outas a step (S10) in the method for producing an inner ring according tothe present embodiment. In this step (S10), a steel stock having an anycomponent composition suitable for induction quenching is prepared, andthe steel stock is forged, turned, etc. to prepare a formed body havinga shape corresponding to a desired shape of the inner ring. Morespecifically, as shown in FIGS. 5 and 6, a formed body corresponding tothe shape of an inner ring having an inner diameter of at least 1000 mmis prepared.

As shown in FIG. 5 and FIG. 6, the formed body is constituted of steeland has an outer circumferential surface having annular groove 19 havinga bottom surface to serve as a raceway surface 11 of the bearing ring.Further, the formed body includes excessive portions 12, 13 in which aregion adjacent to groove 19 extends outwardly of a position indicatedby a dotted line 14 representing an outer circumferential surface of thebearing ring (or inner ring member 3 b). Excessive portion 12 can be setin thickness to, for example, 1 mm or more and 5 mm or less in adirection along the central axis of the formed body. Excessive portion13 can be set in thickness to, for example, 1 mm or more and 5 mm orless in a radial direction perpendicular to the central axis of theformed body.

Then, a normalizing step is carried out as a step (S20). In this step(S20), the formed body prepared in the step (S10) is heated to atemperature of at least an A₁ transformation point and thereafter cooledto a temperature of less than the A₁ transformation point, wherebynormalizing is performed. At this time, a cooling rate in the cooling inthe normalizing may simply be a cooling rate at which the steelconstituting the formed body does not transform into martensite, i.e., acooling rate of less than a critical cooling rate. Hardness of theformed body after the normalizing becomes high when this cooling rateincreases, and becomes low when the cooling rate decreases. Therefore,desired hardness can be imparted to the formed body by adjusting thecooling rate.

Then, referring to FIG. 4, a quench hardening step is carried out. Thisquench hardening step includes an induction heating step carried out asa step (S30) and a cooling step carried out as a step (S40). In step(S30), referring to FIGS. 7 and 8, a coil 121 as an induction heatingcoil is arranged to face the formed body at part of a raceway surface 11(an annular region) which is a surface where a rolling element shouldroll. Note that coil 121 may have any shape.

Then, the formed body is rotated about the central axis, morespecifically, in a direction of arrow α, while a high-frequency currentis supplied to coil 121 from a power source (not shown). Thus, a surfacelayer region of the formed body including raceway surface 11 isinduction-heated to a temperature of at least the A₁ point, and anannular heated region along raceway surface 11 is formed. At the time,the temperature of the surface of raceway surface 11 is measured with athermometer 122 such as a radiation thermometer, and controlled.

Then, in the step (S40), water as a cooling liquid, for example, isinjected toward the whole of the formed body including the heated regionformed in the step (S30), whereby the whole of the heated region issimultaneously cooled to a temperature of not more than the M_(s) point.Thus, the heated region transforms into martensite, and a regionincluding raceway surface 11 hardens. Through the aforementionedprocedure, induction quenching is performed, and the quench hardeningstep is completed.

Then, a tempering step is carried out as a step (S50). In this step(S50), the formed body quench-hardened in the steps (S30) and (S40) ischarged into a furnace, for example, heated to a temperature of not morethan the A₁ point and retained for a prescribed time, whereby temperingis performed.

Then, a finishing step is carried out as a step (S60). In this step(S60), as shown in FIG. 9, by removing excessive portions 12, 13 of theformed body, inner ring member 3 a has its shape adjusted, and otherrequired working such as polishing raceway surface 11 or similarfinishing is carried out. Through the aforementioned process, inner ringmember 3 a constituting an inner ring of a double row tapered rollerbearing is completed. Inner ring member 3 a has an inner diameter of atleast 1000 mm and has a quench-hardened layer homogeneously formed byinduction quenching along raceway surface 11 circumferentially.

Further, inner ring member 3 a has excessive portions 12, 13 removedafter a heat treatment to expose inner ring hardened region 15 b andunhardened region 18 at a region of outer circumferential surface 16adjacent to groove 19 (in FIG. 9, a region of outer circumferentialsurface 16 located closer to the central axis of inner ring member 3 a,as seen at groove 19). By detecting that inner ring member 3 a has outercircumferential surface 16 with inner ring hardened region 15 b andunhardened region 18 formed therein, whether inner ring member 3 a hasbeen produced by using the method for producing a bearing ring accordingto the present disclosure as described above can be easily detected.Whether inner ring hardened region 15 b and unhardened region 18 areformed in outer circumferential surface 16 at a region adjacent togroove 19 can be detected in a conventionally well known method such ashardness measurement. Note that a width of inner ring hardened region 15b in outer circumferential surface 16, that is, a distance from an endof the opening of groove 19 to an end of inner ring hardened region 15b, can be set to 1 mm or more and 10 mm or less. Furthermore, in FIG. 9,only a region of outer circumferential surface 16 located closer to thecentral axis of inner ring member 3 a as seen at groove 19 has bothinner ring hardened region 15 b and unhardened region 18, and a regionof outer circumferential surface 16 located radially outer as seen atgroove 19 exposes only inner ring hardened region 15 b. However, in thepresent disclosure, a region of outer circumferential surface 16exposing both inner ring hardened region 15 b and unhardened region 18may be only the region of inner ring member 3 a located radially outeras seen at groove 19 or may be both the region radially outer as seen atgroove 19 and the region located closer to the central axis as describedabove.

Note that when excessive portions 12, 13 (see FIG. 9) are not formed,and the heat treatment is performed as described above in thatcondition, then, as shown in FIG. 10, inner ring hardened region 15 b isformed in the inner ring member 3 a at a surface facing outer ring 2(see FIG. 2), i.e., the entirety of outer circumferential surface 16.This is because excessive portions 12, 13 do not exist and accordingly,inner ring member 3 a has outer circumferential surface 16 entirelyheated by induction heating.

Furthermore, an assembling step is carried out as a step (S70). In thisstep (S70), inner ring member 3 a produced as described above and innerring member 3 b and outer ring 2 produced in the same manner as innerring member 3 a are assembled together with rollers 6 (FIG. 2) asseparately prepared rolling elements, cage 7 (see FIG. 2), inner ringspacer 4 (see FIG. 2), and the like, whereby double row tapered rollerbearing 1 as shown in FIGS. 1 and 2 is assembled. By the aboveprocedure, the method for producing the double row tapered rollerbearing according to the present embodiment is completed. Furthermore,from another point of view, a method for producing double row taperedroller bearing 1 shown in FIGS. 1 and 2 comprises the steps of:preparing a bearing ring (outer ring 2, inner ring members 3 a and 3 b,inner ring spacer 4 shown in FIG. 2); preparing tapered rollers 6; andassembling double row tapered roller bearing 1 by combining the bearingring and the rollers. The bearing ring (inner ring members 3 a and 3 b)is produced in the method for producing a bearing ring as describedabove.

According to the present embodiment, coil 121 arranged to face part ofraceway surface 11 of the formed body is relatively rotated along thecircumferential direction of the formed body in the step (S30), wherebythe heated region is formed on the formed body. Therefore, it ispossible to employ coil 121 small with respect to the outer shape of theformed body, and the production cost for a quenching apparatus can besuppressed even in a case of quench-hardening a large-sized formed body.In the present embodiment, further, the whole of the heated region issimultaneously cooled to a temperature of not more than the M_(s) point.Therefore, it becomes possible to form inner ring hardened region 15 bwhich is an annular quench-hardened region homogeneous in thecircumferential direction, and residual stress is prevented fromconcentrating on a partial region.

In the step (S30), coil 121 having any shape can be applied in so far asit can heat the formed body by induction heating. For example, anannular coil may be used which covers raceway surface 11 of the formedbody entirely.

It should be noted that although the normalizing step performed in step(S20) is not an essential step in the method for producing a bearingring according to the present invention, carrying out this step allows aformed body of steel to be adjusted in hardness while suppressing quenchcracking.

In this step (S20), hard particles may be sprayed to the formed bodyalong with a gas to perform shot blasting while cooling the formed body.Thus, the shot blasting can be performed simultaneously with air-blastcooling at the time of the normalizing, and scales formed on a surfacelayer portion of the formed body are removed, and reduction ofcharacteristics of inner ring member 3 a resulting from formation of thescales, reduction of thermal conductivity resulting from formation ofthe scales, etc. are suppressed. As the hard particles (a projectionmaterial), metal particles made of steel, cast iron etc. can beemployed, for example.

<Function and Effect of the Above Production Method>

The method for producing a bearing ring according to the presentdisclosure as shown in FIGS. 4 to 9 is a method for producing a bearingring of a double row tapered roller bearing and comprises the steps of:preparing a formed body (S10); forming a heated region (S30); cooing(S40); and removing (S60), as described above. In the step of preparinga formed body (S10), there is prepared a formed body constituted ofsteel and having an outer circumferential surface having annular groove19 having a bottom surface to serve as raceway surface 11 of the bearingring. In the step of forming a heated region (S30), the formed body isinduction heated to form a heated region including the bottom surface ofgroove 19 and heated to a temperature of at least the A₁ point. In thecooling step (S40), the whole of the heated region is simultaneouslycooled to a temperature of not more than the M_(s) point. In the step ofpreparing a formed body (S10), the formed body includes excessiveportions 12, 13 in which a region adjacent to groove 19 extendsoutwardly of a position which should be an outer circumferential surfaceof the bearing ring. In the removing step (S60), the excessive portions12 and 13 are removed from the formed body after the cooling step (S40).

This allows induction heating to be performed to selectively quench aheated region including a bottom surface of groove 19 to serve asraceway surface 11 of inner ring member 3 a constituting a bearing ring,and accordingly, allows the bearing ring to be produced through aprocess simpler than when performing a carburizing heat treatmentaccompanied by an anti-carburization treatment and can also reduce aperiod of time required for the process. This allows the bearing ring tobe produced at a reduced cost.

Furthermore, the quenching process performed in a state where excessiveportions 12 and 13 are present adjacent to groove 19 to be heated canreduce a possibility of overheating or overcooling and thusquench-cracking an end of the opening of groove 19, that is, a (corner)portion connecting an inner peripheral surface of groove 19 and theouter circumferential surface of inner ring member 3 a serving as abearing ring, as would be in a case without excessive portions 12 and13. That is, excessive portions 12 and 13 allow uniform heated andcooled states around groove 19 in the step of forming a heated region(S30) and the cooling step (S40). From a different point of view,excessive portions 12 and 13 can suppress uneven quenching resultingfrom a mass effect around groove 19.

In the method for producing the bearing ring, as described above, theformed body may have an annular shape, as shown in FIG. 7 etc. In stepof preparing a formed body (S10), excessive portions 12, 13 of theformed body may be annularly arranged so as to sandwich groove 19 in thedirection of the central axis of the formed body. In that case,excessive portions 12 and 13 are arranged adjacent to the entirecircumference of groove 19, which can suppress uneven quenchingthroughout groove 19.

In the method for producing the bearing ring, as described above, angleθ (see FIG. 2) that the bottom surface of groove 19 of the formed bodyforms with the central axis in the step of preparing a formed body (S10)may be 40° or more and 50° or less. In that case, in a bearing ring(inner ring member 3 a) of a so-called steep double-row tapered rollerbearing having angle θ falling within such a numerical range asindicated above, a difference easily arises in heated and cooled statesin the quenching process at the outer circumferential surface of thebearing ring between a portion adjacent to groove 19 and a portioncontiguous to the bottom surface of groove 19. Accordingly, the methodfor producing the bearing ring according to the present disclosure isparticularly effective.

A method for producing a double row tapered roller bearing as shown inFIGS. 1 and 2 includes the steps of: preparing a bearing ring; preparingtapered rollers; and assembling a double row tapered roller bearing bycombining the bearing ring and the rollers. Inner ring members 3 a and 3b configuring the bearing ring are produced in the method for producinga bearing ring as described above. Thus double row tapered rollerbearing 1 can be obtained that comprises inner ring members 3 a and 3 bhaving sufficient characteristics without causing a defect such asquench-cracking or inviting an increased production cost.

While an embodiment of the present invention has been described asabove, the embodiment can be variously modified. Further, the presentinvention is not limited in scope to the above-described embodiment. Thescope of the present invention is defined by the terms of the claims,and is intended to include any modifications within the meaning andscope equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

The present embodiment is advantageously applicable to a double rowtapered roller bearing applied to a wind turbine generator, inparticular.

REFERENCE SIGNS LIST

-   -   1: bearing; 20: outer ring; 3 a, 3 b: inner ring member; 4:        inner ring spacer; 5: inner ring; 6: roller; 7: cage; 8: bolt        hole; 9: raceway surface; 10: wind turbine generator; 11:        raceway surface; 12, 13: excessive portion; 14: dotted line; 15        a: outer ring hardened regions; 15 b: inner ring hardened        regions 15 b; 16: outer circumferential surface 17: boundary        portion; 18: unhardened region; 19: groove; 20: rotor head; 22:        main shaft; 25: central axis; 30: blade; 40: speed up gear; 50:        power generator; 60 main shaft bearing; 61: output shaft; 90:        nacelle; 100: tower; 121: coil; 122: thermometer.

The invention claimed is:
 1. A double row tapered roller bearingcomprising: an outer ring serving as a bearing ring having an annularshape; an inner ring disposed on an inner circumferential side of theouter ring and serving as a bearing ring having an annular shape, theinner ring having an outer circumferential surface facing the outer ringand having two rows of grooves having a bottom surface serving as araceway surface; and tapered rollers disposed in the grooves in contactwith the raceway surface of the inner ring and being also in contactwith the outer ring, at the outer circumferential surface of the innerring, a region adjacent to the groove including a hardened regionextending from an inner peripheral surface of the groove to the regionadjacent to the groove, and an unhardened region located at a positionfarther from the groove than the hardened region and being smaller inhardness than the hardened region, the region adjacent to the groovebeing located outside of the groove, and forming a boundary between thehardened region and the unhardended region on the outer circumferencesurface.
 2. The double row tapered roller bearing according to claim 1,wherein an angle that the raceway surface forms with a central axis ofthe inner ring is 40° or more and 50° or less.
 3. A method for producinga bearing ring of a double row tapered roller bearing, comprising thesteps of: preparing a formed body constituted of steel and having anouter circumferential surface having an annular groove having a bottomsurface to serve as a raceway surface of the bearing ring;induction-heating the formed body to form a heated region including thebottom surface of the groove and heated to a temperature of at least anA₁ point; simultaneously cooling a whole of the heated region to atemperature of not more than an M_(s) point, in the step of preparing aformed body, the formed body including an excessive portion in which aregion adjacent to the groove extends outwardly of a position whichbecomes an outer circumferential surface of the bearing ring; andremoving the excessive portion from the formed body after the step ofcooling.
 4. The method for producing a bearing ring according to claim3, wherein the formed body has an annular shape, and in the step ofpreparing a formed body, the excessive portion of the formed body isannularly arranged so as to sandwich the groove in a direction of acentral axis of the formed body.
 5. A method for producing a double rowtapered roller bearing, comprising the steps of: preparing a bearingring; preparing tapered rollers; and assembling a double row taperedroller bearing by combining the bearing ring and the rollers, thebearing ring being produced in the method for producing a bearing ringaccording to claim
 4. 6. The method for producing a bearing ringaccording to claim 4, wherein in the step of preparing a formed body, anangle that the bottom surface of the groove of the formed body formswith the central axis is 40° or more and 50° or less.
 7. A method forproducing a double row tapered roller bearing, comprising the steps of:preparing a bearing ring; preparing tapered rollers; and assembling adouble row tapered roller bearing by combining the bearing ring and therollers, the bearing ring being produced in the method for producing abearing ring according to claim
 6. 8. A method for producing a doublerow tapered roller bearing, comprising the steps of: preparing a bearingring; preparing tapered rollers; and assembling a double row taperedroller bearing by combining the bearing ring and the rollers, thebearing ring being produced in the method for producing a bearing ringaccording to claim 3.